The limited supply and increasing cost of crude oil has prompted the search for alternative processes for producing hydrocarbon products. One such process is the conversion of methanol to hydrocarbons and especially light olefins (by light is meant C.sub.2 to C.sub.4 olefins). The interest in the methanol to olefin process is based on the fact that methanol can be obtained from coal or natural gas by the production of synthesis gas which is then processed to produce methanol.
Processes for converting methanol to light olefins are well known in the art. Initially aluminosilicates or zeolites were used as the catalysts necessary to carry out the conversion. For example, see U.S. Pat. Nos. 4,238,631; 4,328,384, 4,423,274. These patents further disclose the deposition of coke onto the zeolites in order to increase selectivity to light olefins and minimize the formation of C.sub.5 + byproducts. The effect of the coke is to reduce the pore diameter of the zeolite.
The prior art also discloses that silicoaluminophosphates (SAPOs) can be used to catalyze the methanol to olefin process. Thus, U.S. Pat. No. 4,499,327 discloses that many of the SAPO family of molecular sieves can be used to convert methanol to olefins. The '327 patent also discloses that preferred SAPOs are those that have pores large enough to adsorb xenon (kinetic diameter of 4.0 .ANG.) but small enough to exclude isobutane (kinetic diameter of 5.0 .ANG.). A particularly preferred SAPO is SAPO-34.
U.S. Pat. No. 4,752,651 discloses the use of nonzeolitic molecular sieves (NZMS) including ELAPOs and MeAPO molecular sieves to catalyze the methanol to olefin reaction.
Finally, Inui et al. in Applied Catalysis, 58, (1990) 155-163 prepared SAPO-34 materials by what they call the rapid crystallization method. One result of using this procedure is that the SAPO product had crystallites in the range of 0.5 to 2 microns. However, the researchers do not state what the distribution of crystallites is throughout the range. There is also no recognition that smaller crystallites affect catalyst activity or life.
Applicants have found that molecular sieves having the empirical formula (EL.sub.x Al.sub.y P.sub.z)O.sub.2 (hereinafter ELAPO) where EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium and mixtures thereof and x, y and z are the mole fractions of EL, Al and P respectively, and having a small particle size, have a considerably longer life and increased selectivity versus previous catalysts. Specifically the ELAPOs of the instant invention are characterized in that at least 50% of the molecular sieve particles have a particle size less than 1.0 .mu.m and no more than 10% of the particles have a particle size greater than 2.0 .mu.m. Applicants have also found that restricting the total metal (EL) content from about 0.005 to about 0.05 mole fraction further improves the catalytic performance of the ELAPO molecular sieve. There is no indication in the art that reducing the particle size and/or the total metal content of an ELAPO molecular sieve would increase its catalytic performance.